The present invention relates to new aromatic polyimide compositions containing an asymmetrically situated benzamide group that improves polymer solubility.
Rigid-rod polymers are a unique class of thermally stable, aromatic polymers which share the common feature of having a polymer backbone with an all-para geometry and catenation angles of approximately 180 degrees. The only freedom of molecular movement available to this type of structure is the axial rotation, namely, rotations around the carbon-carbon single bonds within the repeat units. Amongst the heterocyclic rigid-rod polymers, the most extensively studied, hitherto, are polybenzoxazole (PBX) family, as represented by polybenzobisthiazoles (PBZT's), polybenzoxazoles (PBO's) and polybenzobisimidazoles (PBI's). The simplest polymers in these families have repeating groups, as follows: ##STR2## The PBX polymers form lyotropic solutions in polyphosphoric acid, methanesulfonic acid, and Lewis acid/nitroalkane. The exploitation of their lyotropic properties has provided nonmetallic materials in the forms of fibers and films with ultra-high specific strength and moduli.
By their molecular geometry, the combinations of aromatic paradiamines and linear aromatic dianhydrides, such as pyromellitic dianhydride (PMDA), naphthlene carboxylic dianhydride (NCDA) and other related higher homologues, constitute another family of rigid-rod polymers, rigid-rod aromatic polyimides (RRPI's), having aromatic heterocyclic structures. These polymers have repeating units as shown, for example, below: ##STR3##
Whereas, as noted previously, PBX polymers form lyotropic solutions in polyphosphoric acid, methanesulfonic acid, and Lewis acid/nitroalkane, RRPI's derived from simple para aromatic diamines show little or no solubility in these acidic solvents. For example, the RRPI derived from PMDA and p-phenylenediamine is only soluble in fuming nitric acid and then only with a substantial degree of polymer degradation.
Because polymer solubility is important in providing more options in the processing and fabrication of RRPI for uses as advanced structural materials and reinforcement component in composites, a common approach to improving their solubilities in organic solvents has been the structural modification of RRPI via pendant chemistry. Thus, extensive backbone substitution with appropriate pendant groups such as, long alkyl chains, phenyl rings, and SO.sub.3 H, has resulted in PMDA-based RRP's being soluble in solvents ranging from chloroform and toluene to N-methyl-2-pyrrolidinone (NMP) and m-cresol to water and sulfuric acid.
I have prepared a series of RRPI's having increased solubility in a variety of solvents.
Accordingly, it is an object of the present invention to provide novel rigid-rod aromatic polyimides.
Other objects and advantages of the present invention will be apparent to those skilled in the art.